Aircraft control apparatus



April 30, 1957 c. G. YATES, JR

AIRCRAFT CONTROL APPARATUS 2 Sheets-Shet 1 Filed Nov. 14, 1951 iu kl H N R. n 2 N M m w l| n5 3x53 m mmkmzozufi 33.8w wzm I l I l l llPIllllllllIl mm O KUW IULLR more: mwEjmE L o mmm o mmm 6518 MP5. 6528 month; m t5 w M. mm

Inventor; Charles. GYQ

His Attorh ey.

April 30, 1957 c. G. YATES, JR

AIRCRAFT CONTROL APPARATUS 2 Sheets-Sheet 2 Filed Nov. 14 1951 PM -Z. UKDWDUKm UCEF" Q a Q a A 1.. mt n 2; me a v A mmws E United States Patent AIRCRAFT CONTROL APPARATUS Charles G. Yates, Jr., Schenectady, N. Y., assig nor to General Electric Company, a corporation New York Application November 14, 1951, Serial No. 256,167

28 Claims. (Cl. 318-489) This invention relates to automatic control apparatus for aircraft and more particularly to such apparatus for controlling the attitude of an aircraft relative to its pitch axlsr 'In the design of control apparatus for aircraft there are two principal factors which must be considered, firstly the standard of performance necessary from the apparatus, and secondly the weight the apparatus adds to the aircraft. Since each pound of control apparatus su'btracts from the pay load of the aircraft, it is desirable that the apparatus weight be kept as small as possible. However, for safety reasons the performance of the apparatus cannot be sacrified in order to save Weight. This is particularly true of automatic pilot apparatus which moves one or more of the control surfaces of the aircraft so as to stabilize it in flight.

Ordinarily automatic pilots contain three control channels or circuits each for controlling the attitude of the aircraft relative to a different axis thereof. Since these channels are electrically separate, a weight saving may be effected in one without adversely affecting the performance of the other two. Thus heretofore there have been various attempts to provide. lightweight control channels of all three types, aileron, elevator, and rudder. These attempts, however, have resulted in poorer performance apparatus requiring the human pilot to trim the aircraft at frequent intervals in order to maintain the aircraft at the desired attitude. Since they require this constant monitoring, these reduced channels actually do not accomplish the primary purpose of autopilots, which is, of course, to provide substantially complete relief for the human pilot. In other words, prior reduced weight channels do not produce the very close control required from relief autopilots.

The conventional, high performance, relief autopilo are, however, of a greater weight than can be tolerated on the latest high speed aircraft; Consequently, there presently exists a great need for control channels of the various types, which combine both light weight and high performance.

It is therefore a principal object of my invention to provide new and improved lightweight, high performance automatic pilot equipment.

It is a more specific object of my invention to provide new and improved automatic pilot equipment for controlling the attitude of an aircraft relative to. its transverse or pitch axis.

'It is another object of my invention to provide automatic pilot equipment for maintaining an aircraft at any predetermined altitude.

It is another object of my invention to provide a new and improved autopilot elevator control channel.

It is still another object of my invention to provide an elevator control channel in which the need for a vertical gyro is obviated.

It is a further object of my invention to provide a lightweight elevator control channel including means for maneuvering the aircraft.

It is still a further object of my invention to provide an improved maneuvering elevator control channel in which the altitude responsive means are automatically adjusted during maneuvering so that the channel is automatically set to, maintain the craft at the altitude attained by the maneuvering.

In carrying out my invention in one form thereof, I utilize altitude responsive signal generating means, rateof-change of altitude responsive signal generating means, rate of-change of pitch responsive signal generating means and elevator position follow-up or repeat-back signal generating means to form a stable, high performance autopilot elevator control channel which will maintain an aircraft at any predetermined altitude. The channel accomplishes this function by combining the outputs of these signal sourcesto actuate elevator positioning means, such as, for example, a servomotor. Since none of its signal sources requires a vertical gyro for control, this new elevator channel is considerably lighter than conventionalhigh performance elevator channels in which vertical gyro controlled signal generating means are needed for satisfactory operation.

If' a maneuvering autopilot is desired, I also include in the elevator channel maneuvering control means in the form of manually controlled signal source. In order to prevent the altitude and altitude rate responsive sources from opposing maneuvering, the manually controlled source is interlocked with means which disable thosesources du-ring the maneuvering. Moreover, automatic means are included which adjust the altitude responsive source so that the channel is automatically set to maintain the aircraft at whatever altitude the human pilot finishes maneuvering the aircraft. Optionally, a time-element canceler may be employed to nullify any steady state signals of the pitch rate and follow-up sources so that undesired trim signals are eliminated during the maneuvering. The canceler, however, is not necessary to the stability of the system.

The features of my invention which are believed to be novel and patentable are pointed out with particularity in the appended claims. The invention itself, however, both as to organization and mode of operation, together with additional objects and advantages thereof, may be best understood by reference to the following description to be taken in conjunction with the accompanying drawing in which:

=Fig. 1 is a simplified schematic diagram in block form of an elevator control channel embodying my invention in'one form; and

Fig. 2 is a diagrammatic representation of an elevator control channel showing my invention in greater detail and embodying maneuvering features.

' Referring to Fig. 1, I have shown therein a new and improved autopilot elevator control channel for maintaining an aircraft (not shown) at any desired altitude. In order to position the aircraft elevators 1 so as to accomplish this level flight control there is provided a servomotor 2 which drives elevators 1 through gears 3. The servomotor 2 is energized from a servo amplifier 4 which iself is connected to be energized from a circuit including a plurality of signal sources or generators in series circuit relation.

A first of these signal sources 5 produces a signal proportional to the magnitude of the displacement of the aircraft from a desired altitude. The polarity of the signal depends upon whether the aircraft is displaced upwardly or downwardly from the desired altitude. Besides this altitude deviation responsive source 5 there is also connected in the circuit an altitude rate responsive source 6. This altitude rate responsive or altitude rate control signal source 6 produces a signal whose magnitude is proportional to the rate of change of altitude of the 3 aircraft and whose polarity is dependent on the direction in which the aircraft is moving from the desired altitude. Various means, for example, a pressure responsive expansible bellows, may be used to control sources 5 and 6.

In addition to these two sources responsive to altitude variations of the aircraft there is also connected in the circuit a pitch rate responsive source 7 which produces a signal in response to the movement of the craft about its transverse or pitch axis. The magnitude of the signal is proportional to the rate of pitch movement, while its polarity is dependent upon the direction of the movement. The pitch rate source 7 may be controlled by any rate of change of pitch responsive device, for example a spring-loaded pitch rate gyro.

Sources 5, 6, and 7 provide the primary signals for maintaining the aircraft at stable flight at the desired altitude, but in order to prevent hunting of the craft about that altitude there are also connected in the circuit servo position follow-up or repeat-back means. A servo follow-up source 8 coupled to motor 2 through gears 9 produces a signal proportional to the displacement of the elevators from a predetermined neutral or null position, and a servo rate source or tachometer 10 coupled directly to the servomotor produces a signal proportional to the rate of movement of the elevators. The signal from source 8 upon any servo displacement is always of a polarity tending to return the servomotor and thus the elevators to their neutral position. The signal from source 10, however, is always of a polarity opposing the movement of the servo causing it, regardless of the direction in which the servo is moving. Sources 8 and 10 operate together to prevent hunting of elevators 1, but it should be understood that the rate responsive source 10 may be omitted without rendering the channel unstable and therefore that I contemplate such an omission among the various modifications of the illustrated channel.

In operation my new and improved elevator control channel not only maintains the aircraft at any desired altitude but also stabilizes the aircraft about its pitch axis. Whenever the aircraft tends to move away from the desired altitude, the altitude deviation source 5 and the altitude rate source 6 produce signals effective to displace elevators 1 in a direction to return the aircraft to the correct altitude. As the elevators move in response to those signals, the servo follow-up 3 and servo tachometer 10 generate signals in order to prevent a greater displacement of the elevators than is necessary. Specifically, the movement of the elevators away from the neutral position continues only until the resultant signal from sources 8 and 10 equals the resultant signal from sources 5 and 6. At that time the elevators are displaced sufficiently to initiate the movement of the aircraft back toward the desired altitude.

As the aircraft so moves, the signal from the altitude rate source 6 thereupon changes polarity and opposes the signal from the altitude deviation source 5 tending to return the elevators to their neutral position. Follow-up source 8, of course, also provides a signal of a polarity to return the elevators to the neutral position. However, due to the opposite direction of elevator movement the servo rate source 10 produces a signal opposing the return of the elevators to the neutral position. In other words, the signals from sources 5 and 10 are now additive to continue the displacement of their elevators while the signals from sources 6 and 8 are additive to eliminate the elevator displacement. As the aircraft moves nearer to the correct altitude, the signal from source 5 becomes smaller allowing the follow-up signal from source 8 and the altitude rate signal from source 6 to return the elevators toward their null position. The signal'from servo rate source 10, opposing this movement and proportional to it, prevents a more rapid elevator movement than is desirable.

Assuming proper circuit and generator design, the

same instant that the elevators reach their null position, with the rate of the movement of each being extremely slow through the last increment of displacement. Thus, the altitude deviation and follow-up signals go to zero at the same time, with the altitude rate and servo rate signals being simultaneously so small as to be negligible. In other words, when the aircraft reaches the desired altitude, there are no actuating signals remaining in the circuit to cause displacement of the elevators. The aircraft is therefore maintained at this altitude without any appreciable hunting, whereby my new and improved lightweight elevator channel provides the very close altitude control required in a relief auto-pilot.

My new channel also acts to stabilize the aircraft about its pitch axis. Whenever the aircraft begins to move about the pitch axis, the pitch rate control 7 produces a signal proportional to this rate of movement. The signal from pitch rate control 7 actuates motor 2 and thereby moves elevators 1 so as to oppose this pitch movement. Since a movement about the pitch axis necessarily results in a change in altitude, signals are also fed to motor 2 from altitude control 5 and altitude rate control 6. These 7 signals together actuate motor 2 sufficiently that the craft is returned to level flight.

' plete autopilot, rudder and aileron channels are also required. My new elevator channel may be used with any rudder and aileron channels which are both electrically and aerodynamically independent of the elevator channel. However, since a great advantage of my new elevator channel is the omission of the heretofore conventional vertical gyro, it is preferable that it be used with an aileron channel which also does not require a vertical gyro. A preferred one of such channels is described and claimed in the copending application of Frank A. Gaynor et al. for Automatic Pilot Equipment," Serial No. 256,168, filed November 14, 1951, now Patent No. 2,705,116, and assigned to the same assignee as the present invention.

Now referring to Fig. 2, I have shown therein my invention in greater detail as embodied in a new and improved elevator control channel in which maneuvering ments of the aircraft about its pitch axis.

control means are included. In Fig. 2, I have shown a control surface 11 which represents the elevator control of an aircraft (not shown) for controlling the move- In order to position elevators 11, I provide servo means, such as a reversible servomotor 12 which is connected to the elevators 11 through gears 13; and I energize the servomotor from a servo amplifier 14 which is actuated by my new and improved elevator control channel. The servomotor 12 responds in direction to the polarity of the output signal from the servo amplifier and its speed is a function of the signal intensity. Included in this elevator channel are a plurality of signal generators or sources 15, 16, 17, 18, 19, 20, and 21, all of which are connected in series circuit relation so that the signal applied to amplifier 14 is the algebraic summation of their individual outputs. The first of the signal sources 15 includes a potentiometer 22 which is connected across a voltage source 23 and has a wiper arm 24 movable in either direction along the potentiometer from coincidence with a fixed midtap 25. A heart-shaped cam 26 is mounted on arm 24 and a spring biased roller 27 bears against the cam to return the wiper arm to coincidence with tap 25 whenever there is no rotational force applied to the arm. The output from the potentiometer 22 is taken across arm 24 and tap 25 and thus varies in magnitude and polarity with the extent and direction of displacement of the arm from coincidence with the tap.

Connected to rotate arm 24 is a shaft 28 on which is keyed an axially movable driving sleeve 29 having a clutch member 30 mounted on its end. This clutch member 30 is movable by sleeve 29 into and out of engagement with a co-operating clutch member 31, as is more fully explained hereinafter, and is here shown in the engaged position. Clutch member 31 which turns wiper arm 24 whenever engaged by member 30 is itself mounted on a shaft 32 driven by an altitude responsive device 33 which is indicated schematically as a bellows 34 fixedly positioned at its one end and coupled at its opposite or free end to a rack 35. The expansion or contraction of bellows 34 in response to changes in altitude moves rack 35 and thereby turns an engaging pinion 36 mounted on shaft 32. Whenever clutch members 30 and 31 are engaged, a turning force is exerted on wiper arm 24 greater than the restraining force exerted by roller 27 and cam 26 so that any changes in altitude result in a movement of wiper arm 24.

However, when clutch members 30 and 31 are disengaged so that there are no altitude responsive forces applied to arm 24, the arm is then returned to coincidence with tap 25, i. e., to the null position by cam 26 and roller 27. Therefore, at no matter what altitude the clutch members are re-engaged, the movement of the wiper arm from the null position is dependent upon the movement of the aircraft up and down from that altitude. In other words, the output from potentiometer 22 is responsive to the deviation of the aircraft from a predetermined altitude, which predetermined altitude is determined by the altitude at which clutch member 30 is moved into engagement with driving clutch member 31.

Since the reaction of some high speed aircraft to a predetermined elevator movement thereof varies at different air speeds, it is desirable that the elevator movement for a certain altitude deviation vary from one air speed to another. Thus, I have optionally included in source 15 means for correcting the signal obtained from potentiometer 22 according to the speed of the aircraft. More specifically, I have provided a gain changer which adjusts the gain, i. e., the magnitude, of the output signal of potentiometer 22 as a function, not necessarily linear, of indicated air speed. This gain adjustment aids in keeping the channel stable throughout the entire speed range of the aircraft. I have illustrated the gain changer schematically as a second potentiometer 37 across which the signal from potentiometer 22 is applied. The output signal of this second potentiometer 37, which signal actually comprises the output of altitude responsive source 15, is taken between a wiper arm 38 movable along the potentiometer 37 from a neutral position and an end tap 39 of the potentiometer.

In order to accomplish the speed correction function, the position of the wiper arm 38 is controlled by an air speed measuring device such as the pressure responsive device 40. As shown schematically, device 40 contains an expansible bellows 41 positioned within an outer casing member 42. The bellows 41 is supplied through an inlet line 43 with air at the dynamic pressure including both the apparent static pressure at any altitude and the velocity head due to the speed of movement of the aircraft, whereas the casing member 42 is supplied through an inlet line 44 wtih air at the apparent static pressure of the altitude. The expansion of the bellows against the force of a restraining spring 45 is then proportional to the velocity head and thus to the speed of movement of the aircraft. The output shaft 46 of the device is arranged to be moved in and out by the expansion and contraction of the bellows and is coupled mechanically to wiper arm 38 so as to control its position along potentiometer 37. Thus, as arm 38 is moved along potentiometer 37 in one direction or the other in response to changes in aircraft speed, more or less of the applied signal from potentiometer 22 is derived between arm 38 and end tap 39. In other words, potentiometer 37 adjusts the output of potentiometer 22 as a function of indicated air speed. More, specifically, the output signal from potentiometer 22 is progressively attenuated by the potentiometer 37 as air speed increases. As mentioned above, the adjusted or corrected signal obtained between arm 38 and tap 39 comprises the output of source 15, which is fed into the servo actuating circuit.

In addition to the output potentiometer 37 of source 15 there is also connected across the output of potentiometer 22 the actuating circuit of a second of the aforementioned signal sources, namely the rate-of-change of altitude responsive source 16. This actuating circuit energizes a reversible motor 47 through an amplifier 48 to drive a tachometer or generator 49, and it includes in series circuit relation amplifier 48, the output of potentiometer 22, one pair of output terminals 50 of tachometer 49, and a signal source 51 whose output is also controlled by motor 47. Tachometer 49 produces a signal proportional to the rate of rotation of'motor 47, whereas source 51 produces a signal indicative of the amount of the displacement of motor 47 from a predetermined neutral position. Source 51 comprises a potentiometer 52 connected across a voltage source 53 and its output is taken between a wiper arm 54 and a fixed potentiometer mid-tap 55, from which the wiper arm 54 is movable in either direction along the potentiometer. The wiper arm is driven from motor 47 through gears 56 so that the magnitude and polarity of the output of source 51 are dependent upon the extent and direction of displacement of motor 47 from the aforesaid neutral position. This circuit so actuates motor 47 that its speed of rotation and thereby that of tachometer 49 are essentially proportional to the rate-of-change of altitude of the aircraft. In fact, the circuit is provided exactly for that purpose so that a signal responsive to the rate-of-change of altitude may be obtained across a second pair of tachometer output terminals 57. In the actuating circuit source 51 is connected in opposition to potentiometer 22 so that whenever a control or actuating signal is supplied from potentiometer 22 to cause rotation of motor 47, the signal thereupon produced by source 51, as a result of the displacement of motor 47, opposes the control signal. Assuming, for example, that potentiometer 22 and source 51, i. e., potentiometer 52, are in their null positions and neglecting for the moment the output of tachometer 49, then if the aircraft changes altitude, a signal is supplied from potentiometer 22 causing the motor to begin to rotate in one direction or the other. As a result of motor 47 leaving its null position, source 51 produces a signal proportional to the displacement, but motor 47 continues to accelerate until the rate of increase of the signal from source 51 equals the rate of increase of the signal from potentiometer 22. Since the inertia of the rotating system is constant, the time required to accelerate the motor to the speed at which the rate-of-change of the follow-up signal equals that of the control signal is proportional to the rate of increase of the control signal. In other words, the more rapidly the altitude is changing, the longer it is before the follow-up signal reaches the same rate-of-change as the control signal, and therefore the greater is the diiference between the control and the follow-up signals when the follow-up signal finally reaches this rate-of-change. Since it is this diiference signal which is impressed upon amplifier 48, thus the greater is the speed at which motor 47 stops accelerating. Then as long as the control signal from potentiometer 22 continues to increase at the same rate, motor 47 runs at the constant speed at which it stopped accelerating, which speed is, of course, proportional to the rate of change of altitude.

If the rate of change of altitude should increase, thereby increasing the rate of change of the control signal, motor 47 also increases speed until it reaches a speed proportional to the new rate-of-change of altitude. Once the rate-of-change of altitude beings to decrease, the motor speed also begins to decrease, although due to inertia it does not decrease quite so fast as the control signal. Thus, as the rate-of-change of altitude goes to zero, the follow-up source 51, in effect, catches up with the control potentiometer 22 so that when the altitude displacement finally reaches a steady value the output of source 51 equals that of potentiometer 22. There is then no signal impressed on amplifier 48 and motor 47 stops rotating.

As motor 47 rotates, it, of course, drives tachometer 49 and the tachometer produces a signal proportional to its speed of rotation across output terminals 50. This signal aids the follow-up signal during an increase in altitude deviation in either direction from the predetermined desired altitude. More specifically, as the aircraft moves away from the desired altitude the tachometer produces a signal across terminals 50 proportional to the rate of movement, which signal opposes the output of potentiometer 22. The tachometer signal is supplied in the circuit to prevent hunting of motor 47, and it does not substantially affect the operation of the circuit as described above. The point at which the motor begins to run at a steady rate is still when the rate of increase of the follow-up signal is equal to the rate of increase of the control signal from potentiometer 22. In fact, as is well known in the art, the end result of including the tachometer in the circuit is that a much smoother operation, and thus a more exact altitude rate signal are obtained.

When the aircraft returns from a displaced position to the desired altitude, a reverse sequence occurs to produce a motor speed proportional to the rate-of-change of altitude, for then the follow-up signal is at all times greater than the control signal from potentiometer 22 since the control signal is decreasing towards zero. Thus the fol-- low-up signal becomes the actuating signal and causes the motor to rotate in the reverse direction. The motor continues to accelerate until the follow-up signal reaches the same rate-of-changc, i. e., rate of decrease, as the control signal from potentiometer 22. The time required by motor 47 to bring the followup signal to the same ratecf-change as the control signal depends as before upon the rate-of-change of the control signal, and thus the motor speed and the tachometer output are still dependent upon the rateofchangc of the control signal. The only difference is that since the follow-up signal is greater than the control signal thereby causing the motor to rotate back to its neutral position, the signal. from the tachometer now opposes the follow-up signal. lilowcver, if the tachometer signal is to prevent hunting of the motor, it should oppose the actuating signal, which in this case is the follow-up signal, and therefore the tachometer signal is of the correct polarity. Thus for changes in altitude in either dricetion, either toward or away from the desired altitude, motor 47 fitlt. thus tachometer 49 rotate at speeds proportional to the rate-of-change of altitude and in a direction dependent upon the direction of change of altitude.

As stated previously the purpose of so rotating tachometer 49 is to obtain a signal across terminals 57 which is ratc-of-change of altitude responsive. lowevcr, since the signals from terminals 57 are employed in the actuating circuit for amplifier 14, the same magnitude signal is not desired from them as from terminals 50. Thcrcfore, the tachometer 49 is here provided with two output windings, and one pair of output terminals is connected across each winding. in an alternating current system. only, a single winding tachometer may be used with one of the associated circuits being coupled to the tachometer output terminals by means of a transformer and with the other circuit being coupled directly to the tachometer output circuit terminals.

Just as the output of potentiometer 22 in source 15 is attenuated as airspeed increases, so is the output of tachometer 49 similarly attenuated in source 16. For that purpose there is connected across terminals 57 a gain changer comprising a potentiometer 58 having a wiper arm 59 movable along it in either direction from a fixed point. The position of wiper arm 59 is controlled by speed measuring device 40 and the output of the potentiometer, i. e., of source 16, is taken between arm 59 and an end tap 60 on the potentiometer. The action of potentiometer 58 so adjusts the output of tachometer 49 that the output signal of ratc-of-change of altitude responsive source 16 at any air speed is of the most desirable magnitude for stable operation of the channel at that air speed. It should be understood, however, that although the gain is adjusted as a function of indicated speed, the function need not necessarily be a linear one, for depending upon the application, a nonlinear adjustment may provide better results.

In addition to this signal from source 16 and the altitude deviation signal from source 15, there is also introduced in the servo circuit a signal responsive to the rate of movement of the aircraft about its transverse or pitch axis, i. e., a rate of pitch responsive signal. This signal is supplied from signal source 17 which is controlled by any rate of pitch responsive device, such as the spring loaded pitch rate gyro 61. The gyro 61 is so positioned on the plane that movement of the plane about its pitch axis tilts the gyro axis in a direction to cause precession against its spring bias. As shown, source 17 includes a potentiometer 62 which i connected across a voltage source 63 and has a wiper arm 64 movable in either direction from coincidence with a fixed mid-tap 65. The Wiper arm 63 is connected to be rotated by pitch rate gyro 61 and thus the output signal which is derived between arm 64 and tap 65 is proportional respectively in magnitude and polarity to the rate of pitch and direction of pitch. of the aircraft, or in other words is rate of pitch responsive.

For best results from the elevator channel this signal should also be adjusted as a function, not necessarily linear, of the speed of the aircraft. Thus a gain changer potentiometer 66 is connected across arm 64 and tap 65 and the output signal of source 17 is taken between the wiper arm 67 and one end tap 68 of potentiometer 66. Wiper arm 67 is coupled to speed responsive device 40 and is movable in either direction along the potentiometer 66. Its resulting movement in one direction or the other, depending on whether the aircraft speed increases or decreases, attenuates the output signal of potentiometer 62 as aircraft speed increases. In other Words, the action of potentiometer 66 results in source 17 supplying a rate of pitch responsive signal in the elevator control channel, which is of the best magnitude for system stability at various air speeds.

In order to prevent hunting of the elevator control surface in response to these various control signals, it is necessary that some elevator position follow-up or repeatback signal be introduced in the circuit. Such a. signal is here obtained from the signal source 18 which includes a potentiometer 69 connected across a voltage source 70. The potentiometer wiper arm 71 is mechanically actuated by the elevator servo 12 through gears 72 so that its movement corresponds to those of the elevator control surface 11; and the follow-up signal output appearing between wiper arm 71 and a fixed tap 73 on potentiometer 69 is variable in magnitude and polarity dependent upon the extent and direction of the movement of wiper arm 71 from coincidence with tap 73.

As is well known in the art, follow-up source 18 is connected in the servo circuit to oppose the control signals causing displacement of elevator control surface 11. In other words, as elevator 11 is displaced from its neutral position in response to any signal or combination of signals from the aforesaid control signal producing means 15, 16,

and 17, source 18 produces a signal tending to return elevator 3 to the neutral position.

Since there is a slight tendency for the follow-up itself to cause a minor hunting of the elevator servo 12, there is also connected in the circuit the signal source 19 which produces a signal proportional to the rate of movement of the elevator servo. This signal source 19 comprises a tachometer which is driven by the servo 12. Tachometer 19 is connected so that its output always opposes the actuating signal causing movement of servo 12, and it thereby prevents servo 12 and thus elevator 11 from moving past their null positions due to rotational inertia. Although tachometer 19 thus improves the operation of the channel, its omission therefrom does not render the channel unstable, and it should be understood that I contemplate such an omission among the modifications of the illustrated channel.

As actuated by the signals from altitude deviation responsive source 15, the rate-of-change of altitude responsive source 16, the pitch rate sounce 17, the follow-up source 18, and the servo rate responsive source 19, the servo 12 moves elevator 11 so as both to maintain the aircraft at the desired altitude and to stabilize it about its pitch axis, as will be more fully explained hereinafter. However, in order to provide means for maneuvering the aircraft while it is under autopilot control, there is also connected in the servo circuit a maneuvering control means comprising the aforesaid signal generator or source 20.

Signal source 20 comprises a potentiometer 74 which is connected across a voltage source 75 and has a wiper arm 76 movable in either direction from a fixed tap 77, the output signal of the source being taken between arm 76 and tap 77 and thus being dependent in polarity and magnitude of the displacement of arm 76 and tap 77. The position of arm 76 is determined by the position of a rotatable shaft 78 to which it is connected. This control shaft 78 comprises one of the two concentric output shafts 78 and 79 of a manually operated joy stick actuator 80 such as is described and claimed in the copending application of Bart H. Snow for Motion Translating Device, Serial No. 245,283, filed September 6, 1951, now Patent No. 2,610,520 and assigned to the same assignee as the present invention. Actuator 88 is operated by a lever or joy stick 81 which is mounted in universal pivots therein, and the actuator translates the motion of the joy stick into rotational movement of the two output shafts 78 and 79. The actuator device has a null or neutral position, i. e., a center position of lever 81 as is shown in the diagram, from which the lever may be displaced in any direction, and each output shaft has a corresponding position from which it may be rotated in either direction.

Actuator 80 is so constructed that one output shaft is rotated in response to movement of lever 81 in a predetermined direction or to any components of lever movement lying in that direction, while the other output shaft is responsive to any lever movement or any component thereof lying at right angles to the above predetermined direction. In other words, the displacements of shafts 78 and 79 from their illustrated null positions are responsive respectively to separate components of the displacement of lever 81 from its null position, which components lie at right angles to each other. Thus, in the illustrated device, shaft 78 is responsive to the components of joy stick movement perpendicular to the plane of the drawing, whereas shaft 79 is responsive to components parallel to the plane of the drawing.

When shaft 78 is in its neutral position, wiper arm 76 contacts tap 77 so that source 20 does not produce a signal. However, when shaft 78 is displaced in either direction from its neutral position by means of lever 81, arm 76 is moved away from tap 77 and a maneuvering control or bias signal is thereupon introduced into the 16 elevator servo control circuit. The direction and eiit'nt of displacement of lever 81, of course, determine the magnitude and polarity of this signal.

However, in order for this signal to cause a change in the elevator position and thereby in the craft attitude, it is preferable that the altitude deviation responsive source 15 and the rate of altitude responsive source 16 be prevented from producing any signal, i. e., be rendered inoperative or disabled. To accomplish this disabling function, there is provided an automatic interlock system which both removes clutch member 30 from engagement with clutch member 31 and places a short across sources. 15 and 16 whenever shaft 78, and thus wiper arm 76, are moved from their neutral positions.

In order to provide means for engaging and disengaging clutch means 30 and 31, the axially movable sleeve 29 on which clutch member 30 in mounted is employed as:

a solenoid armature. Specifically, sleeve or armature 29 is magnetically moved toward member 31 upon the ener-- gization of a solenoid actuating coil 82. A biasing spring- 83, however, moves armature 29 away from member 31, disengaging member 30 from member 31 whenever coil 82 is de-energized.

To energize coil 82, there is employed the aforesaid inter-locking or disabling system. This system or circuit is energized from a grounded D. C. power supply and includes in series circuit relation between the two sides of the supply the solenoid coil 82 and a plurality of switches 84, and 86. The circuit also includes a relay 87 whose normally open spring biased contacts 88 are connected in parallel with switch 84 and whose operating coil 89 is connected in parallel with coil 82. Relay 87 is shown in the diagram in its picked-up or operated position.

In order for coil 82 to be energized, either switch 84 or contacts 88, and both of switches 85 and 86, must be closed. Switches 84 and 85 are manually operable and are spring biased respectively to normally open and normally closed positions. Switch 86, however, is controlled by the position of shaft 78. The movable member of switch 86 is mounted on the linkage connecting shaft 78 and arm 76, and switch 86 is closed only when shaft 78 and thus arm 76 are in their null positions. Whenever joy stick 81 is moved so as to displace arm 76, switch 86 is automatically opened.

Since switch 86 is connected directly to the positive side of the power supply, its opening de-energizes the entire interlock circuit, including coil 82, and thus allows the disengagement of the clutch members. In other words, Whenever arm 76 is moved from coincidence with tap 77 for the purpose of introducing a maneuvering control signal into the elevator control channel, the interlock system automatically removes arm 24 from the control of device 33, whereupon cam 26 as biased by roller 27 returns arm 24 to coincidence with tap 25. Of course, as long as arm 24 and tap 25 are in contact, neither source 15 nor source 16 produces any output signal, which is the desired condition during maneuvering of the aircraft.

Moreover, arm 24 is not automatically returned to the control of altitude indicating device 33 when stick 81 is brought back to its neutral position. Rather, since contacts 88 are open, the interlock system keeps sources 15 and 16 disabled until switch 84 is manually closed by the human pilot. Once switch 84 is closed, reiay 87 picks up, closing contacts 88 and locking in the solenoid coil circuit. This feature requiring the pilot to consciously re-engage the clutch members 30 and 31, when he has completed the altitude maneuvering of the aircraft, is provided to prevent an automatic, momentary re-engagement thereof whenever shaft 78 passes through its neutral position. altitude to another, the pilot might very well operate joy stick" 81 so as to move shaft 78 and arm 76 to their neutral positions one or more times before he man- In changing the aircraft from one l1 ages to level the aircraft olf at the new altitude. It would obviously be undesirable to have clutch members and 31 engage momentarily each time shaft 78 passed through its neutral position.

This result, secured by the opening of switch 86, can also be obtained by opening switch 85. In fact, the purpose of switch is to allow the human pilot to disengage the clutch members without operating the joy stick 81.

To make doubly sure that neither source 15 nor source 16 introduces a signal into the system during maneuvering, I also optionally provide means controlled by the interlock system for shorting out their respective output terminals during maneuvering. Thus, the contacts of a first normally closed relay 91 are connected across the output terminals of source 15 and the contacts 92 of a second normally closed relay 93 are connected across the output terminals of source 16. The relays 9.1 and 93 are shown in their picked-up or operated position. The respective operating coils 94 and 95 of relays 91 and 93 are connected in parallel with coils 82 and 89, and thus are energized and de-energized at the same time as those coils. Thus, whenever switch 86 is opened, the relays drop out, closing contacts 90 and 92 and thereby shorting out sources 15 and 16. As with the clutch solenoid coil 82, the relay operating coils 94 and 95 cannot be reenergized until both switches 86 and 84 are recloscd. Thus, the disabling shorts on sources 15 and 16 are locked on until the human pilot consciously recloscs switch 84 after completing maneuvering of the aircraft.

Besides the various control and maneuvering sources, I have also included in the circuit a pitch rate and follewup canceler signal source 21 which operates as is more fully explained hereinafter, to substantially cancel any steady'state, i. e., non-transient, signals from sources 17, 18 and 1.9. This canceler source 21 is included to nullify the steady-state signals from the follow-up and pitch rate sources, which would otherwise oppose maneuvering of the aircraft. Although the inclusion of the pitch rate and follow-up canceler in the channel provides for a better performance thereof, particularly during maneuvering, it should be understood that the channel is stable and will operate satisfactorily without such a canceler source. Thus, I contemplate among the modifications of the illustrated channel, an elevator control channel in which no pitch rate or follow-up canceler is included.

The illustrated canceler signal source 21 itself comprises a potentiometer 96 which is connected across a voltage source 97 and has a wiper arm 98 movable in either direction from a fixed tap 99, the output signal of the source being taken between arm 98 and tap 99 and thus being dependent in polarity and magnitude upon the direction and magnitude of the displacement of arm 93 from tap 99.

The position of arm 98 is determined by the position of a motor 100 to which it is connected through gears 101. The motor 100 is energized from a motor control unit 102 to rotate in either direction, dependent upon the polarity of the signals applied to control unit 102, and at a rate dependent upon the magnitude of the signals applied to unit 102. As shown in the diagram, control unit 102 is controlled by the signals appearing across points 68 and 103, which signals are the algebraic su1nmation of the output of sources 17, 18, 19 and 21, or in other words, are the combined outputs of sources 17, 18, 19 and 21. The impedance of amplifier 14 is so much greater than the impedance of the various signal sourc that signals from sources 15, 16 and 20 do not appear to any appreciable extent across points 68 and 103. More specifically, although amplifier 14 and control unit 102 are connected in series across sources 15, 16 and .20, sources 17, 18, 19 and 21 form such a low impedance parallel path around control unit 102 that any signal from sources 15, 16 and 20 is dissipated almost entirely across amplifier 14. In order to make control unit 102 and thus canceler source 21 responsive to non-transient signals only, a tachometer generator 104 is included in the input circuit to control unit 102 in the manner described in the copending applications of Charles M. Young, for Airplane Maneuvering System, Serial No. 39,346, and for Autopilot Control System, serial No. 39,347, both filed luly 17, 1948, now Patent No. 2,582,305 and Patent No. 2,654,530, respectively, and assigned to the same assignee as the present invention. Generator 104 is placed in series with the input to control unit 102, so that the actual si nal annlied to control unit 102 at any instant is the LcllC urination of the signal across points 68 and and the output signal from generator 104.

Generator 104 is mechanically coupled with motor 100 to produce output signals varying in polarity and magnitu with the direction and rate of motor motion respec- 'cly, and it is degeneratively electrically connected with respect to the signals across points 68 and 103; in other words, the output of generator 104 is connected in series opposition to the signal applied from points 68 and 103. Assuming that substantially steady-state signals appear across points 68 and 103, motor 100 is thereupon caused to move at a rate and in a direction dependent upon the magnitude and polarity of these si nals. Motion of motor 100 not only rotates generator 104 so that it produces a signal, but also displaces wiper arm 98 so that source 21 produces a signal. The resulting sequence of operation, therefore, is that motor 100 begins to move at a rate proportional to the signal across points 68 and 103 and then slows down as the outputs from generator 104 and signal source 21 oppose the actuating signal. The output from source 21. being connected serially in the servo circuit reduces the signal across points 68 and 103 while the output of generator 104 being applied in the energization circuit fo control unit 102 causes only a portion of the aforesaid reduced signal to be supplied to control unit 102.. When motor 100 moves arm 98 to a position where Jill the El eut giut .n source 21 is equal and opposite to the 'al from s uses 17, 18 and 19, the signal across points 53 and 103, of course, goes to Zero and motor 102 stops rotating. Essentially complete cancellation of steady-state signals from sources 1.7, 18 and 19 is thus obtained after a time interval dependent upon the orig inal ampli ude of the input signal and upon the output characteristics of source 21 and of generator 104.

However, when th' signal from sources 17, 18 and 19 are varying rather rapidly, i. e., are transient signals, cancollation is negligible or very slight because of the generator output characteristics and because of the inability of the motor and associated equipment to respond to rapidly varying control signals. Consequently, transient signals from sources 17, 18 and 19 are impressed on the input of amplifier 14 much as if the pitch rate and followup cancel er were not in the system. As will be explained hereinafter in the action dealing with the operation of the channel, it: necessary that these transient signals be passed by the canceler for they aid in stabilizing the aircraft in flight. I; is only the steady-state, i. e., nontransient, signals which it is desirable to nullify.

In operation, my new and improved elevator control channel both maintains the aircraft at a predetermined altitude and also stabilizes it about the pitch axis. As previously mentioned, upon a movement of the aircraft away from the desired altitude at which clutch members 30 and 31 were engaged, the altitude deviation responsive source 15 supplies a primary control signal, whose magnitude is proportional to the altitude deviation. This signal, of course, actuates serve 12 to move elevators 11 in a direction to return the aircraft to the desired altitude. Moreover, as long as the aircraft is moving away from the desired altitude, the altitude rate responsive source 16 also produces a. signal of a polarity to cause the elevator movement required to return the aircraft to the correct altitude. Since the altitude deviation signal and the altitude rate signal add While the aircraft is moving away from the desired altitude, this results in a much more rapid movement of the elevators and thus a more rapid cessation of such movement than if only an altitude deviation signal were supplied to the servo. As the elevators move, the follow-up source 18 and the follow-up tachometer 19 begin to produce signals opposing such movement. in fact, servo 12 and thus elevators 11 continue moving until the signal output from sources 13 and 19 is equal to that from sources 15 and 16.

By that time, the elevators have been moved far enough to begin to return the aircraft to the desired altitude, and therefore due to the reversal in aircraft movement, the signal from source 16 may be opposing source 15 and aiding sources 18 and 19. This aids in preventing a greater elevator displacement than is absolutely necessary. In any case, as the aircraft moves back to the desired altitude, the follow-up signal from source 18 becomes greater than the altitude deviation signal from source 15, and actuates servo 12 so as to start returning the elevators to their normal or null position. The altitude rate signal also aids in returning the elevators to their null position whereby they are returned rapidly enough to prevent the aircraft from overshooting the desired altitude. However, as the elevators move toward the null position, the follow-up tachometer 19 produces a signal opposing the movement and thereby prevents the elevators from running past the null position, i. e., hunting. It is preferable to eliminate such hunting of the elevators since it could cause a similar hunting of the aircraft. For best results, the aircraft should reach the desired altitude at substantially the same time as the elevators reach their null positions, whereby the altitude deviation signal and the follow-up signal go to zero together. Moreover, the rate of change of each should be so slow as the null position is approached that the altitude rate and the follow-up rate, i. e., the follow-up tachometer, signals are negligible. Then the elevators stop moving at the null position and no hunting occurs. It should be emphasized that during the above transient altitude maintaining sequence, the follow-up signals are varying rapidly so that canceler source 21 does not introduce any appreciable nullifying signals into the circuit. In other words, canceler 21 has little or no eifect on such an altitude maintaining operation of the circuit.

Now, however, suppose that there is a steady downdraft or up-draft on the aircraft whereby a continuing displacement or trim of the elevators is necessary to maintain the aircraft at the desired altitude. Thereupon, the canceler does have an appreciable efiect upon the operation of the circuit. When such a condition exists, and there is no canceler included in the channel, the channel still operates stably to maintain the aircraft in level flight, but the aircraft is, of necessity, displaced from the desired altitude. Specifically upon a steady downdraft or up-draft the aircraft moves oh the desired altitude until the altitude responsive signal is equal to the followup signal. Then a steady elevator displacement occurs which maintains the aircraft in level flight at this displaced altitude. The trim signal from the follow-up is compensated for by an equal and opposite steady-state signal from the altitude responsive source whereby no signal is fed to the servo. However, with a canceler in the circuit, the steady-state follow-up signal is nullified by the canceler, and the signal from the altitude responsive source causes an additional transient movement of the elevators sufficient to return the aircraft to the desired altitude. The altitude responsive signal, of course, decreases as the aircraft nears the desired altitude whereby the elevators return to the correct trim position for maintaining the aircraft at that altitude substantially at the same time as the aircraft itself reaches the altitude. Since the accompanying trim signal is still nullified by the canceler, the elevators thereupon remain at the correct trim position and hold the aircraft at the desired altitude subject to minor or transient variations. These transient altitude variations are, however, corrected for in the 14 manner described above, for the canceler passes transient signals superimposed on a steady-state base. Any tran' sient portions of the follow-up signal necessary to main-' tain system stability during these small altitude deviations are impressed on the servo amplifier much as if the can celer were not in the circuit.

In addition to nullifying steady-state follow-up signals,- the canceler also operates to nullify steady-state pitch-rate signals. As is well known in the art, a gyro-controlled pitch rate source produces a steady-state signal during a banked turn of the aircraft about its vertical axis. if this signal is fed to the elevator servo, it causes an altitude deviation of the aircraft during the turn. Although this is not at all serious since the altitude deviation would be automatically corrected for, as described above, none-- theless in order to prevent it from even occurring, the steady-state pitch rate signal is nullified by the canceler so that it has no effect on the elevator servo. The aircraft is, therefore, not caused to leave the desired altitude, even slightly, during the turn.

It should be understood, however, that my new and improved elevator channel is stable without the inclusion of a rate or a follow-up canceler. Although the canceler provides improved results by eliminating certain steadystate signals in the circuit, a circuit in which the canceler is omitted will operate satisfactorily to maintain the aircraft at a mean desired altitude.

As previously mentioned, in addition to maintaining an aircraft within the desired altitude, my new and improved elevator channel acts to stabilize the craft about it pitch axis. Whenever the aircraft begins to move about its pitch axis, the pitch rate gyro control source 17 introduces a signal into the servo circuit. The signal actuates motor 12 and displaces elevators 11 so as to oppose the movement and stabilize the aircraft about the pitch axis. Since a movement about the pitch axis inherently causes at least a small altitude deviation, the altitude deviation source 15 and the altitude rate source 16 also supply signals of a polarity to return the aircraft to level flight. As above, the servo follow-up 18 and the follow-up tachometer 19 produce transient voltages effective to prevent hunting of the elevators. Since the pitch rate signals are also varying transiently, the canceler 21 again has no detrimental eifect on the operation of the circuit. The transient stabilizing signals are impressed on the servo without there being any nullification by the canceler. Thus, when the channel is maintaining the aircraft in stable flight at a desired altitude, the canceler operates only if the elevators must necessarily be continuously displaced in a trim position.

Another period when the canceler operates due to a continuous displacement of the elevators is when the aircraft is maneuvered in altitude by means of the elevator channel. Whenever the human pilot wishes to maneuver the craft, he introduces a maneuvering control signal into the circuit by means of source 20. As explained above, the activating of source 20 disables the altitude deviation source 15 and the altitude rate source 16, so that only the pitch rate source 17, the canceler source 21, the follow-up source 18 and the follow-up tachometer 19 remain active in the circuit in addition to source 20.

To obtain a certain rate of climb or descent, the human pilot moves joy stick 81 to increase the signal from source 20 in one direction or the other, until the plane reaches that rate. When the plane attains this steady rate, the signal from pitch rate source 17 goes to zero, since the aircraft is no longer moving about the pitch axis. Moreover, since a steady elevator trim position is necessary to continue the steady rate of climb, the follow-up signal from source 18 becomes steady state and the signal from follow-up tachometer 19 goes to zero. The canceler thereupon operates to cancel the steady-state follow-up signal so that there is no signal tending to move the elevators from the necessary trim position to their null position. Thus, once the aircraft attains the desired rate of climb or descent, the human pilot need only return source 20 to its no signal position to continue the aircraft at that rate. When he wishes to level off at the new altitude, he displaces source 20 momentarily in the opposite direction to return the elevators to their neutral position.

If the pilot should wish to loop the aircraft, the operation of the channel would be much as described above, except that a greater elevator displacement would be needed and the pitch rate source would produce a steadystate signal which would be nullified by the canceler. To bring the plane out of the loop, the pilot would need only move source 20 momentarily in the opposite direction.

It should be understood that maneuvering elevator channel embodying my invention may be combined with various types of maneuvering rudder and aileron channels to form complete maneuvering autopilot systems. However, as previously mentioned in order to effect the grcatest Weight saving, these maneuvering elevator channels may be most advantageously employed with maneuvering aileron channels in which the need for a vertical gyro is also obviated. A preferred one of such aileron channels is described in the aforesaid Gaynor et al. application, Serial No. 256,168, now Patent No. 2,705,116.

While the various signal sources included in the servo circuits have been illustrated and described as potentiometer type sources, they have been so depicted primarily for the sake of clearness in understanding the invention, and it will be readily appreciated that selsyn-type inductive instruments such as shown in Patent 2,464,629 Young may utilized in place of the potentiometers if desired. ll potentiometers are used, the voltage sources shown may be D. C. or A. C. and in practice, a common source would be used if A. C. sources were employed. Also, the generating means controlled by the canceler motor may comprise selsyn or potentiameter units and associated equipment for obtaining a voltage proportional to speed rather than conventional generators as shown. The various generator output characteristics with respect to speed may be selected or adjusted to secure the optimum rate-of-change of the canceler signal, and the various motor control units or the motors themselves may be adapted to prevent excessive or insufficient rateof-change of the canceler signals.

Thus, While in accordance with the patent statutes, I have described what at present are considered to be the preferred embodiments of my invention, it will be obvious to those skilled in the art that numerous alterations and modifications may be made therein without departing from the invention, and is, therefore, aimed in the apded, claims to cover all such equivalent variations as all within the true spirit and scope of this invention.

What l claim new and desire to secure by Letters elative pitch axis thereof, signal responsive actuating me: nected to move said elevator, rate responsive in for generating a continuous signal proportional to the rate-ofchange of craft attitude relative to said. pitch axis, means for generating a continuous signal proportional to the altitude deviation of said craft from a predetermined altitude, means for producing a continuous signal proportional to the rate-of-change of altitude of said c it, means for generating a continuous signal in resp o displace ment of said elevator from a predeterminet. ric mal pot and 1 cans coupling said signals to our actuating means.

2. In a control apparatus for an aircraft having an elevator adapted to control the craft attitude relative to the pitch axis thereof, signal responsive actuating means connected to move said elevator, rate responsive means for generating a. first continuous signal proportional to the rate-of-change of craft attitude relative to said pitch axis, means for producing a second continuous signal 18 proportional to the altitude deviation of said craft from a predetermined altitude, means for producing a third continuous signal proportional to the rate-of-change of altitude of said craft, means for generating a fourth continuous signal in response to displacement of said elevator from a null position, means for adjusting the magnitudes of said first, second, and third signals as a function of the speed of flight of said craft, and means coupling the adjusted signals and said fourth signal to energize said actuating means.

3. In a control apparatus for an aircraft having an elevator arranged to maneuver said aircraft about its pitch axis, a servomotor arranged to movably position said elevator, rate responsive means for producing a continuous signal proportional to the rate of movement of said aircraft about said pitch axis, means for generating a continuous signal proportional to the altitude deviation of said aircraft from a predetermined altitude, means for producing a continuous signal proportional to the rate-of-change of altitude of said aircraft, followup means for producing a continuous signal proportional to the displacement of said elevator from a predetermined normal position, and means coupling said signals to actuate said servomotor.

4. In a control apparatus for an aircraft having an elevator arranged to maneuver said aircraft about its pitch axis, a servomotor arranged to movably position said elevator, rate responsive means for producing a first signal continuously proportional to the rate of movement of said aircraft about said pitch axis, means for generating a second signal continuously proportional to the altitude deviation of said aircraft from a predetermined altitude, means for producing a third signal continuously proportional to the rate-of-change of altitude of said aircraft, follow-up means for producing a fourth signal continuously proportional to the displacement of said elevator from .a predetermined normal position, means for adjusting the magnitudes of said first, second and third signals in accordance with the speed of flight of said aircraft, and means coupling the adjusted signals and said fourth signal to actuate said servomotor.

5. In a control apparatus for an aircraft having an elevator for controlling the craft attitude relative to the pitch axis thereof, a servomotor connected to move said elevator, rate responsive means for generating a continuous signal proportional to the rate of movement of said craft about said pitch axis, means for producing a continuous signal proportional to the altitude deviation of said craft from a predetermined altitude, means for producing a continuous signal proportional to the rateof-change of altitude of said craft, means for producing a signal proportional to the displacement of said elevator from a predetermined normal position, means for producing a signal proportional to the rate of movement of said elevator, and means coupling said signals to actuate said servomotor.

6. In a control apparatus for an aircraft having an elevator for controlling the craft attitude relative to the pitch axis thereof, a servomotor connected to move said elevator, rate responsive means for generating a first continuous signal proportional to the rate of movement of said craft about said pitch axis, means for producing a second continuous signal proportional to the altitude deviation of said craft from a predetermined altitude, means for producinga third continuous signal proportional to the rate-of-change of altitude of said craft, means for producing a fourth signal proportional to the displacement of said elevator from a predetermined nor mal position, means for producing a fifth signal proportional to the rate of movement of said elevator, means for adjusting the magnitudes of said first, second and third signals as a function of the speed of flight of said craft, and means coupling the adjusted signals and said fourth and fifth signals to actuate said servomotor.

7. In a control apparatus for an aircraft having an elevator arranged to maneuver said craft about the pitch axis thereof, signal responsive actuating means connected to move said elevator, means for producing a first signal proportional to the rate of movement of said craft about said pitch axis, means for producing a second signal proportional to the altitude deviation of said craft from a predetermined altitude, means for producing a third signal proportional to the rate-of-change of altitude of said craft, means for producing a fourth signal in response to the displacement of said elevator from a predetermined normal position, means coupling said signals to energize said actuating means, and time element means connected to render substantially steady-state values of said first and fourth signals ineffective to energize said actuating means.

8. In a control apparatus for an aircraft having an elevator arranged to maneuver said craft about the pitch axis thereof, signal responsive actuating means connected to move said elevator, means for producing a first signal proportional to the rate of movement of said craft about said pitch axis, means for producing a second signal proportional to the altitude deviation of said craft from a predetermined altitude, means for producing a third signal proportional to the rate-of-change of altitude of said craft, means for producing a fourth signal in response to the displacement of said elevator from a predetermined normal position, means for adjusting said first, second and third signals in magnitude as a function of the speed of flight of said craft, means coupling the adjusted signals and said fourth signal to energize said actuating means, and time element means connected to render substantially steady-state values of the adjusted first signal and said fourth signal ineffective to control said actuating means.

9. In a control apparatus for an aircraft having an elevator adapted to control the craft attitude relative to the pitch axis thereof, an autopilot control channel comprising a servomotor connected to move said elevator, means for producing a first signal proportional to the altitude deviation of said craft from a predetermined altitude, means for producing a second signal proportional to the rate-ofchange of altitude of said craft, means for producing a third signal proportional to the rate of movement of said craft about said pitch axis, means for producing a fourth signal proportional to the displacement of said elevator from a predetermined normal position, means for producing a fifth signal proportional to the rate of movement of said elevator, means coupling said signals to actuate said servo means, and time element means connected to render substantially steady-state values of said third, fourth, and fifth signals ineffective to actuate said servo means.

10. In a control apparatus for an aircraft having an elevator for controlling the craft attitude relative to the pitch axis thereof, elevator control apparatus comprising a servomotor arranged to movably position said elevator, means for producing a first signal proportional to the altitude deviation of said craft from a predetermined altitude, means for producing a second signal proportional to the rate-of-change of altitude of said craft, means for producing a third signal proportional to the rate of movement of said craft about said pitch axis, means for producing a fourth signal proportional to the displacement of said elevator from a null position, means for producing a fifth signal proportional to the rate of movement of said elevator, means for adjusting the magnitudes of said first, second and third signals in accordance with the speed of flight of said craft, means cou- 11. In a control apparatus for an aircraft having an elevator for maneuvering said aircraft about its pitch axis, elevator control apparatus comprising servo means arranged to movably position said elevator, attitude stabilizing means for producing a signal proportional to the rate of movement of said aircraft about said pitch axis, altitude maintaining means including means for producing a signal proportional to the altitude deviation of said aircraft from a predetermined altitude and means for producing a signal proportional to the rate-of-change of altitude of said aircraft, follow-up means for producing a signal in response to the displacement of said elevator from a predetermined normal position, manually controlled maneuvering means for producing a signal, means coupling said signals to actuate said servo means, and means interlocked with said maneuvering means for disabling said altitude maintaining means in response to signal generating actuation of said maneuvering means.

12. In a control apparatus for an aircraft having an elevator for maneuvering said aircraft about its pitch axis, elevator control apparatus comprising a servomotor arranged to movably position said elevator, attitude stabilizing means for producing a signal proportional to .the rate of movement of said craft about said pitch axis,

altitude maintaining means including means for producing a signal proportional to the deviation of said craft ,from a predetermined altitude and means for producing .a signal proportional to the rate-of-change of altitude of said craft, follow-up means for producing a signal in response to the displacement of said elevator from a predetermined normal position, manually controlled maneuvering means for producing a signal, means for adjusting the magnitudes of the signals from said attitude stabilizing means and said altitude maintaining means as a function of the speed of flight of said aircraft, means coupling the adjusted signals and the signals from said follow-up means and said maneuvering means to actuate said servomotor, and means interlocked with said maneuvering means for disabling said altitude maintaining means in response to signal generating actuation of said maneuvering means.

13. In a control apparatus for an aircraft having an elevator for maneuvering said aircraft about its pitch axis, elevator control apparatus comprising a servomotor arranged to movably position said elevator, attitude stabilizing means for producing a signal proportional to the rate of movement of said aircraft about said pitch axis, altitude maintaining means including means for producing a signal proportional to the altitude deviation of said aircraft from a predetermined altitude and means for producing a signal proportional to the rate-of-change of altitude of said aircraft, follow-up means including means or producing a signal proportional to the displacement of said elevator from a predetermined normal position and means for producing a signal proportional to the rate of movement of said elevator, manually controlled maneuvering means for producing a signal, means coupling said signals to actuate said servomotor, and means interlocked with said maneuvering means for disabling said altitude maintaining means Whenever said maneuvering means is producing a signal.

14. In a control apparatus for an aircraft having an elevator for maneuvering said aircraft about its pitch axis, elevator control apparatus comprising a servomotor arranged to movably position said elevator, attitude stabilizing means for producing a signal proportional to the rate of movement of said craft about said pitch axis, altitude maintaining means including means for producing a signal proportional to the deviation of said craft from a predetermined altitude and means for producing a signal proportional to the rate-of-change of altitude of said craft, follow-up means including means for producing a signal proportional to the displacement of said elevator from a predetermined normal position and means for producing a signal proportional to the rate of move- 4 i9 ment of said elevator, manually controlled maneuvering means for producing a signal, means for adjusting the magnitudes of the signals from said attitude stabilizing means and said altitude maintaining means as a function of the speed of flight of said aircraft, means coupling the adjusted signals and the signals from said follow-up means and said maneuvering means to actuate said servomotor, and means interlocked with said maneuvering means for disabling said altitude maintaining means whenever said maneuvering means is producing a signal.

15. In a control apparatus for an aircraft having an elevator adapted to control the craft attitude relative to the pitch axis thereof, maneuvering elevator control apparatus comprising a servomotor connected to move said elevator, attitude stabilizing means for producing a signal proportional to the rate-ofchange of said craft attitude relative to said pitch axis, altitude maintaining means including means for producing a signal proportional to the altitude deviation of said craft from a predetermined altitude and means for producing a signal proportional to the rate-of-change of altitude of said craft, follow-up means for producing a signal in response to the displacement of said elevator from a normal null position, manually controlled maneuvering means for producing a maneuvering control signal, means coupling said signals to actuate said servomotor, time element means connected to render substantially steady-state signals from said follow-up means and said attitude stabilizing means ineffective to actuate said servomotor, and means interlocked with said maneuvering means for disabling said altitude maintaining means whenever said maneuvering means is producing a signal.

16. In a control apparatus for an aircraft having an elevator adapted to control the craft attitude relative to the pitch axis thereof, maneuvering elevator control apparatus comprising a servomotor connected to move said elevator, attitude stabilizing means for producing a signal proportional to the rate-of-change of said craft attitude relative to said pitch axis, altitude maintaining means including means for producing a signal proportional to altitude deviation :of said craft from a predetermined altitude and means for producing a signal proportional to the rate of change of altitude of said craft, follow-up means for producing a signal in response to the displacement of said control surface from a normal null position, manually controlled maneuvering means for producing a signal, means for adjusting the signals from said attitude stabilizing means and said altitude maintaining means as a function of the speed of flight of said craft, imeans coupling the adjusted signals and the signals from said follow-up means and said maneuvering means to actuate said servomotor, time element means connected to render substantially steady-state signals from said follow-up means and said attitude stabilizing means ineffective to actuate said servo motor, and means interlocked with said maneuvering means for disabling said altitude maintaining means in response to signal producing actuation of said maneuvering means.

17. In a control apparatus for an aircraft having an elevator adapted to maneuver said craft about its pitch axis, maneuvering elevator control apparatus comprising a servomotor connected to move said elevator, attitude stabilizing means for producing a signal proportional to the rate of movement of said craft about said pitch axis, altitude maintaining means including means for producing a signal proportional to the altitude deviation of said craft from a predetermined altitude and means for producing a signal proportional to the rate-of-ch ange of altitude of said aircraft, follow-up means including means for producing a signal proportional to the displacement of said elevator from "a normal null position and means for producing a signal proportional to the rate of movement of said elevator, manually controlled maneuvering means for producing a signal, means coupling said signals to actuate said servoinotor, time element means connected 20 to render substantially steady state signals from said attitude stabilizing means and aid follow-up means ineffective to actuate said servo-motor, and means interlocked with said maneuvering means for disabling said altitude maintaining means whenever said maneuvering means is -'duc r a signal.

iii. in a control apparatus for an aircraft having an elevator for maneuvering the craft about its pitch axis, maneuvering elevator control apparatus comprising a servomotor connected to move said elevator, attitude stabilizing means for producing a signal proportional to the rate of: \OVfilTlBl'll of said craft about said pitch axis, altitude maintaining means including means for producing a signal proportional to the altitude deviation of said craft from a predetermined altitude and means for producing a signal responsive to the rate-of-change of altitude of said craft, follow-up means including means for producing a signal proportional to the displacement of said elevator from a normal null position and means for producing a signal proportional to the rate of movement of said elevator, manually controlled maneuvering means for producing a signal, means for adjusting the magnitudes of the signals from said attitude stabilizing means and said altitude maintaining means as a function of the speed of flight of said craft, means coupling the adjusted signals and the signals from said follow-up means and said maneuvering means to actuate said servomotor, time element means connected to render substantially steady-state signals from said attitude stabilizing means and said follow-up means ineffective to actuate said servomotor, and "means interlocked with said maneuvering means for disabling said altitude maintaining means Whenever said maneuvering means is producing a signal.

19. In a high performance servo control system for positioning the elevator of an aircraft to both maintain the aircraft at a desired altitude and provide pitch stabilization at said altitude, reversibly operating means for positioning said elevator, altitude sensing means for energizing said positioning means with a continuous signal proportional to aircraft deviation from a desired altitude, altitude rate sensing means energizing said positioning means with a continuous signal proportional to the rate of change of aircraft altitude, attitude rate sensing means responsive to aircraft instability for energizing said positioning means with a continuous signal proportional to the rate of change of aircraft attitude relative to the pitch axis, foll0w-up means for additionally energizing said positioning means with a continuous signal proportional to elevator movement, and means coupling said signals to actuate said reversibly operating means, whereby said altitude sensing means and altitude rate sensing means jointly enable the servo system to maintain the craft at a fixed altitude, and said attitude sensing means and altitude rate sensing means jointly enable the maintenance of pitch stabilization at said fixed altitude.

20. In a high performance servo control system for positioning the elevator of an aircraft to both maintain the aircraft at a desired altitude and provide pitch stabilization at said altitude, reversibly operating means for positioning said elevator, altitude sensing means for energizing said positioning means with a continuous signal proportional to aircraft deviation from a desired altitude, altitude rate sensing means for energizing said positioning means with a continuous signal proportional to rate of change of aircraft altitude, attitude rate sensing means responsive to aircraft instability for energizing said positioning means wth a continuous signal proportional to the rate of change of aircraft attitude relative to the pitch axis, means for adjusting the signal amplitudes of said altitude sensing means, altitude rate sensing means, and attitude sensing means as a function of the speed of flight of said aircraft, follow-up means for addition-ally energizing said positioning means with a continuous signal proportional to elevator movement, and means coupling said signals to actuate said reversibly operating means, whereby said altitude 21 sensing means and altitude pate sensing means jointly enable the servo system to maintain the craft at fixed altitude, and said attitude sensing means and altitude rate sensing means jointly enable'the maintenance of pitch stabilization at said fixed altitude.

21. In a high performance servo control system for positioning the elevator of an aircraft to both maintain the aircraft at a desired altitude and provide pitch stabilization at said altitude, reversibly operating means for positioning said elevator, altitude sensing means for energizing said positioning means with a continuous signal proportional to aircraft deviation from a desired altitude, altitude rate sensing means for energizing said positioning means with a continuous signal proportional to rate of change of aircraft altitude, attitude rate sensing means responsive to aircraft instability for energizing said positioning means with a continuous signal proportional to the rate of change of aircraft attitude relative to said pitch axis, means for adjusting the signal magnitudes of said altitude sensing means, altitude rate sensing means, and attitude sensing means as the function of the speed of flight of said aircraft, followup means for additionally energizing said positioning means with a continuous signal proportional to elevator movement, additional follow-up means for energizing said positioning means with a signal proportional to the rate of change of elevator movement, and means coupling said signals to actuate said reversibly operating means, whereby said altitude sensing means and altitude rate sensing means jointly enable the servo system to maintain the craft at fixed altitude, and said attitude sensing means and altitude rate sensing means jointly enable the maintenance of pitch stabilization at said fixed altitude.

22. In a control apparatus for an aircraft having an elevator adapted to control the craft attitude relative to the pitch axis thereof, signal responsive actuating means connected to move said elevator, a rate gym for generating a continuous signal proportional to the rate-of-change of craft attitude relative to said pitch axis, means for generating a continuous signal proportional to the altitude deviation of said craft from a predetermined altitude, means for producing a continuous signal proportional to the rate-of-change of altitude of said craft, means for generating a continuous signal in response to displace ment of said elevator from a predetermined normal position, and means coupling said signals to energize said actuating means.

23. In a control apparatus for an aircraft having an elevator arranged to maneuver said craft about the pitch axis thereof, signal responsive actuating means connected to move said elevator, means for producing a first signal proportional to the rate of movement of said craft about said pit-ch axis, means for producing a second signal proportional to the altitude deviation of said craft from a predetermined altitude, means for producing a third signal proportional to the rate-of-change of altitude of said craft, means for producing a fourth signal in response to the displacement of said elevator from a predetermined normal position, means for adjusting the magnitude of said first, second, and third signals as a function of the flight speed of said craft, means coupling the adjusted signals and said fourth signal to energize said actuating means, and means responsive to said adjusted first and fourth signals and energizing said actuating means with a cancelling signal equal and opposite to the steady-state values thereof, said means including a signal generator and a time delayed motor circuit for actuating said generator and being energized by the algebraic sum of said adjusted first signal and fourth signal, in addition to said generator signal.

24. In a control apparatus for an aircraft having an elevator arranged to maneuver said craft about the pitch axis thereof, signal responsive actuating means connected to move said elevator, a rate gyro for producing a first signal proportional to the rate of movement of said craft about said pitch axis, means for producing a second signal proportional to the altitude deviation of said craft from a predetermined altitude, means for producing a third signal proportional to the rate-of-change of altitude of said craft, means for producing a fourth signal in response to the displacement of said elevator from a predetermined normal position, means coupling said signals to energize said actuating means, and time element means connected to render substantially steady-state values of said first and fourth signals ineffective to energize said actuating means.

25. In a. control apparatus for an aircraft having an elevator arranged to maneuver said craft about the pitch axis thereof, signal responsive actuating means connected to move said elevator, a rate gyro for producing a first signal proportional to the rate of movement of said craft about said pitch axis, means for producing a second signal proportional to the altitude deviation of said craft from a predetermined altitude, means for producing a third signal proportional to the rate-of-change of altitude of said craft, means for producing a fourth signal in response to the displacement of said elevator from a predetermined normal position, means for adjusting said first, second and third signals in magnitude as a function of the speed of flight of said craft, means coupling the adjusted signals and said fourth signal to energize said actuating means, and time element means connected to render substantially steady-state values of the adjusted first signal and fourth signal ineffective to control said actuating means.

26. In a control apparatus for an aircraft having an elevator for maneuvering said aircraft about its pitch axis, elevator control apparatus comprising servo means arranged to movably position said elevator, a rate gyro for producing a signal proportional to the rate of movement of said aircraft about said pitch axis, altitude maintaining means including means for producing a signal proportional to the altitude deviation of said aircraft from a predetermined altitude and means for producing a signal proportional to the rate-of-change of altitude of said aircraft, followup means for producing a signal in response to the displacement of said elevator from a predetermined normal position, manually controlled maneuvering means for producing a signal, means coupling said signals to actuate said servo means, and means interlocked With said maneuvering means for disabling said altitude maintaining means in response to signal generating actuation of said maneuvering means.

27. In a control apparatus for an aircraft having an elevator arranged to maneuver said craft about the pitch axis thereof, signal responsive actuating means connected to move said elevator, means for producing a first signal proportional to the rate of movement of said craft about said pitch axis, means for producing a second signal proportional to the altitude deviation of said craft from a predetermined altitude, means for producing a third signal proportional to the rate-of-change of altitude of said craft, means for producing a fourth signal in response to the displacement of said elevator from a predetermined normal position, means coupling said signals to energize said actuating means, and means responsive to said first and fourth signals for energizing said actuating means with a cancelling signal equal and opposite to steady-state values thereof, said means including a signal generator and a time delayed motor circuit for actuating said generator and being energized by the algebraic sum of said first, fourth and generator signals.

28. In a control apparatus for an aircraft having an elevator for maneuvering said aircraft about its pitch axis, elevator control apparatus comprising servo means arranged to movably position said elevator, attitude stabilizing means for producing a signal proportional to the rate of movement of said aircraft about said pitch axis, altitude maintaining means including means for producing a signal proportional to the altitude deviation of said aircraft from a predetermined altitude and means for producing a signal proportional to the rate-of-change of altitude of said aircraft, follow-up means for producing a signal in response to the displacement of said elevator from a predetermined normal position, manually controlled maneuvering means for producing a signal, means coupling said signals to actuate said servo means, means interlocked with said maneuvering means for disabling said altitude maintaining means in response to signal generating actuation of said maneuvering means, and means responsive to said attitude stabilizing means and said follow-up means for energizing said servo means with a cancelling signal equal and opposite to steady-state values thereof, said means including a signal generator and a time delayed motor circuit for actuating said generator and being energized by the algebraic sum of said altitude stabilizing means signal, and said follow-up means signal, and said generator signal.

References Cited in the file of this patent UNITED STATES PATENTS Isserstedt Oct. 30, Nisbet Jan. 14, Moseley Jan. 14, Noxon Dec. 2, Young Mar. 15, Hofstader May 9, Rossive Jan. 27, Redemske May 22, Young Jan. 15, Halpert Feb. 19, Divoll Apr. 15, Nash May 27, Young Oct. 28, Strother Dec. 2, Hanna May 12, 

